Osmia bicolor nests

Rob Parker of the Suffolk Naturalists' Society sent me [Adrian Knowles] the following anecdote regarding an Osmia bicolor nest which he had previously observed being covered by pine needles as part of their "tent making" behaviour:

"I returned to the King's Forest on this morning's Dingy Skipper search, and found my O. bicolor nest had grown substantially since my last visit 11 days ago. We carefully lifted the pagoda of pine needles off, and found the snail shell had a large pine kernel over the top of it, so that it was quite impossible to see the shell from outside. Also, it would have been well-screened from sunlight. We looked at the shell, which weighed very little, and was part-filled with very fine soil and an entry-exit hole, from which several ants emerged."

Is this a case of ant predation of the nest or were they just helping themselves to a previously raided nest? The fact that the shell was still under its tent suggests that it was not attacked by birds etc.